Magnetic properties of nanocrystalline CoFeNbB alloys

Abstract
The influence of alloy composition and heat treatment on the structure and magnetic properties of rapidly quenched CoFeNbB alloys were studied. The saturation magnetization, J_s, the magnetostriction constant. λs, and the Curie teniperature. T_c, of the as-quenched amorphous alloys decrease with increasing Nb or B content (3 < x_Nb < 7. 8 < x_B < 15 at.%). Both J_s, and λs, of CoFeNb₇B₉ show maxima at about 29 at.% of Co. The crystallization of all the alloys takes place in two steps. On the first step bcc-Fe(Co) grains develop in the amorphous matrix. With increasing Nb content their grain sizes substantially decrease. A nanocrystalline structure with the grain size of 10 - 15 nm is obtained for x_Nb = 7 at.%. The saturation magnetization increases on nanocrystallization. The magnetostriction constant, λs, is positive for the as-quenched as well as nanocrystalline alloys. Its magnitude first decreases and then remains constant or increases with further annealing. A slight decrease of the coercive field. H_c,, at the early stage of nanocrystallisation is followed by a subsequent gradual increase. The best soft magnetic properties were observed for the nanocrystalline Co₂₅Fe₅₉Nb₇B₉ alloy. The following parameters J_s = 1.58 T. λs and H_c = 26 5 A/m. were obtained in this alloy after annealing at 600°C.